If your present wheelset is using 700x32 I'd say that you've already got your rail trail set. Pehaps a switch to a more knobby cyclocross style of tire?

That leaves the road option. 25 to 28mm road smoothies roll SOOOOOOO nice you may want to consider a setup more like this for your second wheelset.

Assuming rim brakes you will want to measure your present rims and try to find the same width rims at the braking faces on your second wheelset. That will be the first step to making this a true "Plug n' Play" setup.

Next is to pick out a wheelset that has the same dropout spacing as your bike. You'll need to measure your frame to find that. Road standard right now is 130 and MTB is 135 mm. Hybrids may be either. See Sheldon's site for how to measure the spacing...http://sheldonbrown.com/gloss_sp-ss.html#spacing

No need to worry about bearings and axle. That all comes with the wheelset. And provided you buy a set with Shimano hubs the QR skewers come with them as well.

So that takes care of the buying part. Now to do the fine tuning so this is truly a Plug n' Play deal....

To make it so that the shifting works with both setups you'll want to shim either the axle cones or the cassete back and forth a hair so that both cassettes sit in exaclty the same side to side position in relation to the drive side dropout. This will avoid the need to totally retune the rear derrailleur with each change. It's more than just the barrel adjuster. You'd have to alter the limit screws as well. So it's best to shim the cassettes so that they both sit in the exact same position when you mount the wheels. This will involve begging some of the shim washers that come with cassettes from your LBS to get them to where they are matched to within around .5 mm's or .010 to .015 inches. That'll be close enough.

Once the hubs are on and shifting matched then the final part is to match up the wheel dishing so the rims run in exactly the same positions. This will avoid the need to retune the brakes with each switch. If you're not comfy with altering the dish by working with the spokes then it's best to take it into a shop for this. They'll true all four wheels so they run the same for you and perhaps your present set needs some touching up anyway.

So it's more than just buying stuff to do it right but once you jump the hoops you'll have a 30 second swap to go from speedy street riding to tough trail adventuring.

One final item. Because you'll be switching wheelsets and nothing ever matches perfectly I'd really recomend a plastic spoke guard or "dork disc". If the chain should ever decide to take an ender off the inside of the big rear sprocket the guard will really save the day. Otherwise you're gambling on perhaps ripping out all the drive side spokes en mass and walking back to the house or car carrying a pile of rubble. The spoke guard turns such an event into just a mildly annoying need to get off and reset the chain. Highly recomended.

One other thing you want to budget for is a rear cog set for these second wheels. If you are really getting this to quickly swap wheels, then you don't want to have to get your chain whip and cog remover out to swap the cassette. This is relatively time consuming and dirty for a quick wheel change. Otherwise, if you have the time to do this kind of swap, it would be cheaper to just swap the tires.